Electroacoustical apparatus



July 1, 1941. H. F. OLSON 2,247,563

ELECTROACOUSTICAL APPARATUS Original Filed Oct. 51, 1938 F5954- J W 1 5W a 5504 m/vr 1 z a 1 5 J Z 4 F550 A/T 3nventor Patented July 1, 1941 UNHTED STATES PATENT amen "FECE.

ELECTROACOUSTICAL APPARATUS Harry F. Olson, Haddon I-leights, N. J., assignor to Radio Corporation of America, a corporation of Delaware 14 Claims.

This invention relates to electro-acoustical apparatus and more particularly to a sound translating device for translating acoustical Waves into electrical variations, the invention having for its primary object the provision of an improved and highly directional sound translating device or microphone by means of which it will be possible to pick up sounds over very long distances, and the present application being a division of my copending application Serial No. 237,960, filed October 31, 1938.

More particularly, it is an object of my present invention to provide an improved directional microphone which is especially suitable for use in sound motion picture recording, television pickup, large stage productions in radio broadcasting, sound reinforcing system-s, etc., where it is desired to keep the microphone out of the field of action and where it is, therefore, necessary to place the microphone at a considerable distance from the sound source.

Another object of my present invention is to provide an improved directional microphone as aforesaid which has a directional characteristic independent of the frequency of the sound picked up thereby.

Still another object of my present invention is to provide an improved directional microphone as aforesaid which has a relatively small angle of pickup.

A further and important object of my present invention is to provide an improved directional microphone as aforesaid which is capable of discriminating against certain frequencies and in favor of other frequencies.

It is also an object of my present invention to provide an improved ultra-directional microphone for the purpose set forth which is compact in construction, easily portable, and highly efficient in use.

In accordance with my present invention, I provide a sound pickup system which comprises a large number of pipes of different length connected to a common junction, which, inturn, is connected to one side of an element adapted to convert the sound waves which travel down the pipes into corresponding electrical variations, the said element being terminated on its other side by an acoutical resistance. The length of the line, that is, the distance between the longest and shortest pipe, determines the directional characteristic of the system. To render the system discriminatory with respect to certain predetermined frequencies, the pipes may be provided with more or less enlarged portions either adjacent their open ends or at suitable points along their lengths and intermediate their ends.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of several embodiments thereof, when read in connection with the accompanying drawing, in which Figures 1 to 3, inclusive, show various modifications of microphones formed according to my present invention, and

Figures 4 to 6, inclusive, are vector diagrams showing the outputs of microphones in accordance with my present invention.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown, in Fig. 1, a microphone comprising a plurality of open-ended, tubular elements I, 2, 3, 4, 5 and 6 of progressively varying length coupled to or having a common junction at a casing 1 within which is vibratably mounted a suitable conductor, such as a crimped ribbon element 8, for vibration in a magnetic field in well known manner. The tubular elements I to 6, inclusive, are of uniformly progressively varying length, so that their free, or open pickup, ends lie along a straight line. The tubular elements I to 6 are preferably all parallel and are arranged to direct acoustical waves onto the front surface of the vibratile member, or ribbon 8, the diameters of all the tubular elements I to 6 being preferably the same throughout the major portions of their lengths, and small compared to the length of the ribbon 8.

Coupled to the rear of the casing or junction 1 is a long tube or pipe 9 filled with tufts of felt m, the pipe 9 with its felt tufts H) constituting an acoustical resistance which terminates the ribbon 8 and which has a value substantially equal to the surge resistance of the tubes l to 6, and large compared to the mass reactance of the ribbon 8. Preferably, the total cross-sectional area of the tubular elements I to 6, inclusive, is substantially equal to the cross-sectional area of the large pipe 9. This very largely minimizes reflection at the junction or casing I, particularly when the acoustic impedance of the ribbon 8 is small compared to the acoustic resistance offered by the pipe 9. If the impedance of the ribbon 8 is comparable to the surge resistance of the tubular elements I to 6, then the vector sum of the impedance terminating the ribbon 8 and the impedance of the ribbon 3 itself should be equal to the surge impedance of the tubular elements I to 6.

If a plane sound wave traveling from left to right in a direction parallel to the axis of tube I to is considered, it will be noted that sound enters each of the small tubes I to 6 and travels down to the ribbon 8 and into the long, damped pipe 9. In going from the small tubes I to 6 into the large pipe 9, the sound Waves must, of course, excite the ribbon 8. In the case under consideration, all the outputs of the tubes i tov 0 are in phase and may be represented by the vector diagram shown in Fig. 4 wherein the numerals I to 0, inclusive, correspond to the tubes I to 6 of Fig. 1

Suppose, now, that sound incident to an angle 0 with respect to the axis of the microphone is considered. Let the instantaneous pressure contributed by the tube I at the ribbon 8 be given by p sin %(cb-l) (1) where where d is the distance between the open ends of adjacent tubes I and 2. imilarly, the pressure contributed by the tube 3 will be and so forth for each of the other pipes, the distance between the open ends of the tubes 2 and 3 also being equal to d.

If it is assumed that the distance d between the ends of any two adjacent tubes is of a wave length and that the angle 0 is 60, then the vectors will be as shown in Fig. 5. From this figure, it will be noted that the response is considerably attenuated as compared to the case represented by the vector diagram of Fig. l, in connection with which it was assumed that the angle 0 is equal to 0. Let it be assumed, now, that the distance d between the ends of any pair of adjacent tubes is /6 wave length but that the angle 0 is 90. In such case, the vectors will be as shown in Fig. 6, from which it will be noted that the response is zero.

If the tubular elements I to 6, inclusive, are formed of uniform diameter throughout their lengths, there may be some attenuation of sound therein at the high frequencies. For example, in a system employing pipes of diameter, I have found that the attenuation at 10,000 cycles is 2.5 db. per foot of length, while at 1,000 cycles, the attenuation is only 0.7 db. per foot of length for the same size pipe. This loss can be counteracted by attaching to the open or pickup ends of the tubes I to 6 or by forming the ends thereof with enlarged portions II, I2, I3, I4, I5 and I6, respectively, to constitute small horns. The characteristic of each of the small horns II to i6 can be made so that these horns will increase the pressure delivered to any of the small tubes I to 6 at the high frequencies without effecting the performance at low frequencies. This increase in pressure offsets the losses incurred in each of the small pipes due to attenuation. In an actual model which was constructed, a small exponential horn 1 A in length with mouth and throat diameters of A and respectively, coupled to a pipe was used. Such a horn gave about 6 db. gain at 10,000 cycles and compensated for the loss in the associated pipe.

In some cases, the reverse action of that obtained by the modification of Fig. 1 is desirable, that is, it may be desirable to attenuate the higher frequencies, and perhaps, even, to provide progressive cutoff of the higher frequencies. For this purpose, the free or pickup ends of the tubular elements I to 6, inclusive, may be provided With inertances El, 22, 23, 24, 25 and 26, respectively, by reducing the free ends of the tubes I to 6, preferably progressively and in correspondence with the length of the tubes I to 6, the shortest tube I having the longest inertance 2|, the longest tube 6 having the shortest inertance 26, etc. The inertance is given by 7 Pl ll1' S where p is the density of the air, I is the length of the small tube, and S is the cross-sectional area thereof.

The impedance is given by iwM. Then, each pipe consists of a small inertance connected to the end. The inertance becomes progressively smaller as one goes from the tube I to the tube 6. This means that the pressure not only becomes attenuated at the higher frequencies, but the amount of attenuation becomes greater as the inertance is made larger. By a suitable choice of constants, the length of the system can be made to be inversely proportional to the frequency. Therefore, the directional characteristics will be inversely proportional to the frequency.

The system of Fig. 3 shows another method of shortening the effective length of the line with frequency. In this modification, wherein only five of the tubular elements are illustrated, the tubes I to 5 are provided with expanded sections or enlargel volumes 3I, 32, 33, 34 and 35, respectively, intermediate their lengths to provide acoustic capacitances, the volumes 3| to 35 being progressively larger in going from the shortest tube I to the longest tube 5. In the equivalent electrical circuit, the capacitance of the volume under consideration appears as a shunt capacitance across the resistance. The acoustic capacitance is given by C2 where p V is the volume in cubic centimeters, p is the density of the air, and c is the velocity of the sound in centimeters per second.

The impedance of any tubular element is given by Where A is the area.

Thus, it is possible to attenuate the pressure delivered by any pipe at the higher frequencies.

By a suitable choice of constants, the length of the system can be made inversely proportional to the frequency.

From the foregoing description, it will be apparent to those skilled in the art that I have provided a novel microphone which is highly directional and the characteristics of which are practically independent of the frequency, the microphone being made discriminatory either in favor of or against the higher frequencies depending upon the manner in which the tubular elements of the line are enlarged or reduced in diameter, as the case may be, at suitable points intermediate their ends. It will be noted, of course, that the small horns H to 16 of Fig. 1 provide enlarged portions for each of the tubes 1 to 6, inclusive, beginning with the throats of these horns and continuing to the mouths thereof. Similarly, the inertances 2| to 26 of Fig. 2 provide reduced portions from the free or open ends thereof up unto the points where they merge with the tubes 1 to 6 so that, in each of the modifications disclosed, the tubular elements each include portions intermediate their ends which are of relatively small diameter and other portions which are of relatively large diameter.

Although I have shown and described several modifications of my invention, I am aware that many other modifications thereof are possible and that many changes may be made in the particular modifications shown and described without departing from the spirit of my invention. I therefore desire it to be understood that I do not wish "to limit myself except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In electro-acoustical apparatus, the combination of a vibratile member adapted to be actuated by acoustical waves, a plurality of openended tubular elements adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one side of said member whereby to direct acoustical waves to said member, and means providing an acoustical resistance coupled to the opposite side of said member and terminating said member, said tubular elements including means differentially responsive to different sound frequencies for discriminating with respect to certain predetermined frequencies.

2. In electroacoustical apparatus, the comblnation of a vibratile member adapted to be actuated by acoustical waves, a plurality of openended tubular elements of progressively varying length adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one side of said member whereby to direct acoustical waves to said member, and means providing an acoustical resistance coupled to the opposite side of said member and terminating said member, said tubular elements including means differentially responsive to different sound frequencies for discriminating with respect to certain predetermined. frequencies.

3. In electro-acoustical apparatus, the combination of a vibratile member adapted to be actuated by acoustical waves, a plurality of openended tubular elements adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one side of said member whereby to direct acoustical Waves to said member, and means providing an acoustical resistance coupled to the opposite side of said member, said tubular elements having certain portions intermediate their ends of relatively large diameter and certain other portions intermediate their ends of relatively small diameter.

4. The invention set forth in claim 3 characterized in that said tubular elements are of progressively varying length and characterized further in that said portions of differing diameters vary in correspondence with the varying lengths of said tubular elements.

5. In electro-acoustical apparatus, the combination of a vibratile member adapted to be actuated by acoustical waves, a plurality of openended tubular elements adapted to receive acoustical waves and transmit said waves along their lengths, said elements being coupled to one side of said member whereby to direct acoustical waves to said member, said tubular elements each having a diameter that is small compared to the length of said member and each including means differentially responsive to different sound frequencies for discriminating with respect to certain predetermined frequencies, and means providing an acoustical resistance coupled to the opposite side of said member and terminating said member.

6. The invention set forth in claim 5 characterized in that the diameters of said tubular elements are different at certain portions of their lengths than at other portions thereof.

'7. The invention set forth in claim 5 characterized in that the free ends of said tubular elements are of different diameters than the remainder of said elements.

8. The invention set forth in claim 5 characterized in that said tubular elements are of progressively varying lengths, and characterized further in that the free ends of said tubular elements are constituted by small horns.

9. The invention set forth in claim 5 characterized in that said tubular elements are of progressively varying length and characterized further in that the free ends of said tubular elements are constituted by portions of relatively smaller diameter than the remaining portions of said tubular elements, said end portions constituting inertances.

10. The invention set forth in claim 5 characterized in that said tubular elements are of progressively varying length and characterized further in that the free ends of said tubular elements are constituted by portions of relatively smaller diameter than the remaining portions of said tubular elements, said end portions constituting inertances, and said inertances being of progressively varying length in correspondence with the varying lengths of said tubular elements.

11. The invention set forth in claim 5 characterized in that said tubular elements are of progressively varying length and characterized further in that the free ends of said tubular elements are constituted by portions of relatively smaller diameter than the remaining portions of said tubular elements, said end portions constiting inertances, and said inertances being of progressively varying length in correspondence with the varying lengths of said tubular elements, the longest of said tubular elements being provided with the shortest of said inertances and vice versa.

12. In electro-acoustical apparatus, the combination of a vibratile member adapted to be actuated by acoustical waves, a plurality of tuaeterized in that said capacitances are of progressively varying volume in correspondence with the varying lengths of said tubular elements.

14. The invention set forth in claim 12 characterized in that said capacitances are of progressively varying volume in correspondence with the varying lengths of said tubular elements, the capacitances having the smallest volume being formed in the shortest of said tubular elements 10 and vice versa.

HARRY F. OLSON. 

